An effective sideboard can often be the difference between winning and losing in Magic the Gathering Arena Best of 3 matches.

Reid Duke has old, but fantastic guide to sideboarding posted.

Here are some excerps with a link to the full article at the bottom.

Sideboards win tournaments. Because your sideboard cards can be more specialized—pinpoint focused for a certain task or matchup—they're often your most powerful tools. Sometimes, sideboarding can be the most important factor in determining how two decks will match up against one another. Building and using your sideboard well will be crucial to your tournament success.

What is your opponent doing that gives you trouble? How can you counter it?

You sideboard with the goal of making your deck better suited for a matchup. What simpler way is there to accomplish that goal than to bring in the perfect answers for your opponent's threats?

What can you do to threaten your opponents deck?

On the flip side, you can also use your sideboard to find a new threat that your opponent will (hopefully) struggle with. You might go about this in a number of different ways.

First, you might simply add more threats in an attempt to overload your opponent's answers. Imagine, for example, that you face a control deck that features very few creatures. During sideboarding, you get to take out some of your ineffective creature removal and increase your concentration of threats, which ought to be tremendously helpful.

Hate cards. What are they and why have them?

Answering problems and adding threats are examples of versatile ways to use your sideboard slots. Over the course of a long tournament, you're likely to turn to these cards often, as they'll serve as minor upgrades in a lot of matchups. However, another approach is to look for hate cards—single cards that are extremely effective at beating (hating out) a particular deck, color, or strategy.

How to build your sideboard. A daunting task, but Reid explains this very, very well.

It can be quite difficult to hone in on the perfect fifteen cards. One way to start the process is to employ the elephant method. Legendary deck builder and Pro Tour Hall of Famer Zvi Mowshowitz explains the elephant method as:

"Writing out ideal realistic lists for all matchups and then trying to make the unique cards in those lists add up to 75 cards before deciding on the specific 60 for the main deck and the specific fifteen for the sideboard."

When you employ the elephant method, you're thinking of your deck as a complete 75-card unit. First, you consider what you'd like your deck to look like after sideboarding in each of the matchups you expect to face. Next, you make sure you have the proper number of cards to bring in and take out in each matchup. Finally, you construct your deck and sideboard accordingly. In short, you look at the big picture, and your sideboard is as important to the big picture as your main deck is.